That is no reflection on Moroney. Labour's women's affairs spokeswoman has given as good as she has got in her running argument with Finance Minister Bill English, which was sparked by her measure making it on to the parliamentary agenda.

She has made the most of the good fortune of her bill being pulled out of the hat in the pre-Easter ballot. That might never have happened. Her bill was competing with nearly 60 other private member's measures for three slots on the parliamentary order paper.

Whether that outcome is ultimately Labour's good fortune is another matter.

The question Labour leader David Shearer must be asking himself is whether Labour is sending the right message to voters in pushing a measure of less than pressing urgency but one which will cost an extra $150 million a year.

If the political debate stays focused on the social benefits of paid parental leave, all well and good. If the debate becomes solely one of affordability, Labour has problems.

What has made things tricky for National and Labour is that Moroney drafted the bill in 2009 when circumstances were different and with a different target in mind.

The bill seems to have been motivated in part to embarrass Pansy Wong, probably the least effective Minister of Women's Affairs in the portfolio's history.

Had it come up for debate in the last Parliament, National would have only needed Act's votes to block its progress.

Instead, it languished for close to three years in the pool of private member's measures waiting for a Lotto-winning-like call-up.

During that time, the Government's fiscal position deteriorated substantially, election changes left National needing more than just Act's support to halt Opposition measures, and Labour elected a new leader who will reverse the party's drift to the left and march it back to the centre.

Shearer believes one reason Labour lost the last election was that its spending promises left voters unconvinced the party was capable of displaying fiscal restraint.

He reiterated that point in his first major speech as leader last month, saying any government he led was going to be "thrifty".

Now he finds himself with a $150-million-a-year commitment slung around his neck - one that centrist-minded voters may take as a sign Labour has not changed.

Shearer is obliged to back Moroney's bill. It is Labour policy after all. There is one compensating factor. Labour desperately needs to shore up its left flank from incursion by the Greens. Highlighting an extension of paid parental leave may help to do that - if only temporarily.

The question is whether gains Labour makes on its left flank will outweigh losses on its right flank.

National's private polling is said to show voters are averse to more Government borrowing and debt.

Carefully avoiding attacking the concept of paid parental leave, English has instead relentlessly sought to exploit Labour's fiscal vulnerability by citing Moroney's bill as compelling evidence that Labour is addicted to spending money the Government does not have.

Moroney has countered by arguing the extension to 26 weeks will be phased in over three years, by which time - according to English's own forecasts - the Government's books will be back in surplus.

She has gone on the front foot, accusing National of being "undemocratic" in using the financial veto to block her bill.

That charge cannot be sustained. The veto was introduced to ensure the opposition could not frustrate a minority government's constitutional right to retain control over its Budget. Without it, effective government would be well nigh impossible.

According to Parliament's rules, a government can exercise the veto if it believes an opposition initiative or amendment to legislation would have "more than a minor impact" on its fiscal position.

The veto has been exercised 36 times since 1999, mainly to block Opposition amendments to bills.

The difference this time is that National is blocking a complete opposition bill which looks certain to make it to select committee stage and would quite possibly have made it into law with the help of the votes of National's allies, the Maori Party and United Future's Peter Dunne.

Moroney says National is frustrating the "will of Parliament".

While the veto overrides any such notion, National's rapid resort to it looked heavy-handed.

It looked like an attempt to stymie debate before it had really begun. It was a response that said National was unwilling to consider any compromise.

National believes any concessions would not have squared with its tight fiscal stance and its paramount priority of getting back into surplus as soon as possible.

To use the Beehive lingo, it had to "manage expectations" before they got out of hand.

A Cabinet Office circular from 2007 shows National was perfectly within its rights to foreshadow its intention to exercise the veto.

But had it held off doing so, it might have stopped Labour capturing the moral high ground.

National, however, was blinkered by two things - the quest for a Budget surplus and its unwillingness to concede even a millimetre of ground to Labour.

Its problem now is that the veto can be exercised only at the final third reading stage of a bill's progress through Parliament.

This enables a government to kill any opposition amendments which have been inserted during the preceding committee stage.

National may now try to convince its allies not to back the bill's introduction on the grounds it is a dead duck and there is no point in wasting House time debating it.

That may be a forlorn hope. National should instead take the obvious message from this episode - that when it comes to passing or blocking legislation, it is now even more of a minority Government than it really likes to admit.

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Herald political correspondent John Armstrong has been covering politics at a national level for nearly 30 years.
Based in the Press Gallery at Parliament in Wellington, John has worked for the Herald since 1987.
John was named Best Columnist at the 2013 Canon Media Awards and was a previous winner of Qantas media awards as best political columnist.
Prior to joining the Herald, John worked at Parliament for the New Zealand Press Association. A graduate of Canterbury University's journalism school, John began his career in journalism in 1981 on the Christchurch Star. John has a Masters of Arts degree in political science from Canterbury.